376 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



others perlitic. Parts of it are lithoidal. Before the valley commences to 

 canyon, at an altitude of about 7,900 feet, the stream cuts its way through 

 lithoidal rhyolite, well laminated and much contorted, with most of the 

 layers standing in a vertical position. Below this the stream continues to 

 cut through dark-gray porphyritic rhyolite, which is lithoidal and finely 

 banded, while on the top of the western wall at the upper end of the 

 canyon proper the rock is glassy and beautifully spherulitic in irregularly 

 shaped forms, inclosing grains of obsidian, and producing the appearance 

 of large axiolites on the surface of the rock in a maimer already described. 

 It also contains small hollow spherulites, distinctly fibrous (1946), and 

 passes into highly vesicular to pmniceous lithoidal rock, light bluish gray 

 in color (1945). 



Half a mile above the mouth of Bechler Canyon the rock exposed in 

 the stream bed is dense lithoidal rhyolite (1949). It forms the bed of the 

 stream up to near Colonnade Falls, which is about a mile above the mouth 

 of the canyon. Here the rhyolite is overlain by a horizontal sheet of basalt 

 which is at the same altitude as that of the great basalt sheet in Falls River 

 Basin, a tongue of which must have flowed up into the canyon. It gives rise 

 to Colonnade Falls, where the stream drops 60 feet from a ledge of rock into 

 a basin partly inclosed by a semicircular wall of columnar basalt. At the 

 base of this wall the large vertical columns are 30 feet high; they pass up 

 into irregularly cracked basalt, which at the top is massive and vesicular, 

 forming a layer which projects over the face of the wall. The wet columnar 

 rock forms a dark background for the free-falling water with its rainbowed 

 mist, while the banks, kept moist by the shifting spray, are covered with a 

 luxuriant growth of ferns on the one side and of flowers on the other. 



A hundred yards upstream is Iris Falls, about 40 feet high, of 

 different character. It is broad and is broken by large masses of rock at 

 its base. The rock is porphyritic rhyolite, which is a later flow than the 

 basalt, and must have been a small one in the bottom of the canyon. The 

 lower portion of this later flow of rhyolite is glassy, being a spherulitic 

 obsidian (1961), and is brecciated with inclosed masses of older rhyolite. 

 It grades upward into lithoidal rock which is light gray colored (1948). 

 This passes up, at the top of the falls, into black, glassy, and spherulitic 

 forms again. Half a mile farther upstream is another waterfall, of 50 feet, 

 having the same general character as the last. It cuts through the later 



